Researchers at the University of California San Diego School of Medicine have found that adolescents who begin using cannabis may experience slower development in key thinking and memory skills compared to their peers, according to a new long-term study.
The findings, published April 20 in Neuropsychopharmacology, draw on data from more than 11,000 participants enrolled in the Adolescent Brain Cognitive Development Study—the largest ongoing study of brain development in youth in the United States.
Lead author Natasha Wade, an assistant professor of psychiatry, said adolescence represents a critical window for brain growth. “Teens who start using cannabis aren’t improving at the same rate as their peers,” she explained, noting that even small early differences can accumulate over time and affect learning and daily functioning.
The research team tracked 11,036 children from ages 9 and 10 through ages 16 and 17, monitoring both cognitive performance and substance use. To improve accuracy, the study combined self-reported cannabis use with biological testing—including hair, urine and saliva samples—capable of detecting drug exposure over varying timeframes.
Across multiple domains such as memory, attention, language and processing speed, teens who used cannabis showed slower progress over time. In some cases, those same individuals performed similarly—or even slightly better—than peers before initiating cannabis use. However, their development appeared to plateau as use began, while non-using peers continued to improve.
The study also examined specific cannabis compounds. In a subset of participants, those with evidence of exposure to tetrahydrocannabinol (THC) showed declines in memory performance over time. By contrast, teens with evidence of cannabidiol (CBD) did not exhibit the same pattern, though researchers cautioned that sample sizes were smaller.
Importantly, the authors emphasized that the study does not establish a direct causal relationship. While they controlled for factors such as family background, mental health and other substance use, additional variables could contribute to the observed differences.
Still, researchers say the findings carry weight. During adolescence, even modest changes in cognitive development can influence academic performance and everyday decision-making.
The team plans to continue following participants into early adulthood to better understand long-term effects, including how the timing and frequency of cannabis use may shape brain development.
Read the whole article from UC San Diego here.
